Nowadays magnetic clutches are used in pump drives in which a can separates an outer rotor and an inner rotor from one another. The outer rotor, driven by a motor and equipped with magnets, drives the inner rotor, also equipped with magnets, by the magnetic field generated by the magnets.
In the past, the magnets of both rotors have been retained in position by gluing or by another positive connection to the rotors. The attachment of the magnets is required to prevent the magnets from being released from the rotor by centrifugal force in the case of the inner rotor, and to prevent the slipping or sliding of the magnets in the case of the outer rotor.
The patent literature discloses a large number of methods of anchoring magnets reliably on rotors. For example, German Patent Document 6,641,165 discloses a magnet cage for small dynamos in which the magnet bars are regularly spaced around the armature axis parallel therewith, each of the magnet bars having on its back a flat or rounded bead interrupted at each edge by a ground-in channel extending as far as the back of the magnet. The resulting lug or bead formed at the magnet bar end engages in a recess of the cap, thus retaining the magnet bars firmly in the radial and axial directions. This construction has the disadvantage of requiring a relatively large amount of fabrication equipment, since the magnet bars must be specially shaped and ground and tolerances must be observed.
British Patent Specification 2,052,319 also discloses an attachment of magnets to a rotor. To this end the rotor is formed with recesses into which the rectangular magnets are inserted. Then a member is fitted around the rotor and the magnet and bent over, said member engaging by its ends around the inner side of the rotor and thereby preventing the magnets from being lost in the radial direction. This construction has the disadvantage that there is a relatively large radial distance between the surface of the magnet and the surface of the member engaging around the rotor, so that the outer effect of the magnet is reduced. Moreover, filling members are needed and a considerable accuracy of fit is required, since the filling members and the magnets might otherwise be either unmountable or would knock backwards and forwards in the rotor and therefore wear more rapidly. It is also impossible to jacket the magnets with plastic, since no openings or channels are provided for the injection of the liquid plastic.
French Patent 1,162,656 also discloses a system for radially securing the magnets of a rotor. In this case also a ring encloses the rotor. To prevent the dislodging of the magnets by the centrifugal forces, this French Patent suggests that the wall thickness of the circular ring should be so increased in the zone of the magnets that they bear via their surfaces firmly against the inner side of the ring in the radial direction. It calls for a very elaborate manufacturing technology and it is disproportionately expensive to make such a ring.
A further possibility is to cement the magnets in matching recesses in the rotors. However, such gluing requires extensive preparational and cleaning work and is also a manufacturing step which cannot be automated. Moreover, in dependence on centrifugal force loading, the glued connection is not reliable in the case of the inner magnet rotor and is also not completely resistant to temperature and chemicals. For this reason magnets should be glued in rotors only in exceptional cases.
Since for reasons of protection (e.g. against corrosion) the inner magnet rotor of magnetic clutch in magnetic clutch pumps should be jacketed with plastic, since it is surrounded by the medium to be conveyed, the glued connection must also survive the plastic processing operation with its high thermal and mechanical loading, something which usually results in a thermal overloading of glue, especially in the case of plastics resistant to high temperatures.
Positive connections of the magnet to the rotor are moreover as a rule very expensive, since the magnets can then be produced only with the use of special tools or their shape must be correspondingly ground.